1. Field of the Invention
The present invention relates to an implantable middle ear hearing device having a tubular vibration transducer to drive a round window. More particularly, the implantable middle ear hearing device of the present invention is designed to input sound to the round window opposite an oval window in an inner ear using the tubular vibration transducer having a unique structure that does not attenuate the magnitude of a signal, particularly, in a high frequency band.
2. Description of the Related Art
In general, about 15% of the world population has mild to severe hearing loss, and about 1 to 2% of the people with hearing loss cannot hear better using a hearing device that transmits sound through the external ear canal. People with hearing loss wears hearing device inserted into the ear, which makes the people very uncomfortable before they are accustomed to the inserted device. It is also inconvenient to attach or detach the device.
To cope with these problems, some countries such as USA, Japan and Germany have variously researched new types of hearing devices, which can be directly implanted in the temporal bone, since 1980s. Some of these types of implantable hearing devices are commercially used.
The implantable hearing device generally includes a microphone, a device body, which manages signal-processing and controls other components of the device, and a vibrator, which converts electric signals into mechanical vibrations to vibrate an ossicle. Of these components, the vibrator is a miniature device that reproduces sound by directly vibrating a very sensitive portion, such as an ossicle or an opening to a cochlear duct. Thus, the vibrator is regarded as the most important component of a middle ear implantable hearing device.
Various types of vibration transducers, which have been researched and developed up to the present, can be divided into an electromagnetic vibrator type and a piezoelectric vibrator type.
Of these vibrators, the piezoelectric vibrator transmits vibration by installing an anchor in the outer wall of the middle ear cavity such that one end of the piezoelectric bimorph comes in contact with an ossicle such as a stapes. This scheme, which was first proposed by Yanagihara, professor of Ehime University, Japan, has been widely adopted by various models such as Envoy by St. Croix Medical (USA) and TICA by Implex AG (Germany). Related patents include, for example, U.S. Pat. No. 6,726,618, U.S. Pat. No. 6,585,637 and U.S. Pat. No. 6,540,662.
However, this scheme has drawbacks in that the operation of implanting a piezoelectric transducer inside a middle ear cavity is complicated and time consuming.
A transducer using the electromagnetic vibrator has been commercially distributed by the proposal of Ball of Symphonix Devices, Inc. of USA. This type of transducer may include a so-called floating mass transducer, which is fitted to the incus of the ossicle using a clamp, and related patents are, for example, U.S. Pat. No. 5,800,336 and U.S. Pat. No. 5,913,815.
However, the electromagnetic vibrator type transducer can be implanted only in the presence of the ossicle, and has to be clamped for a long time, which may disadvantageously burden the ossicle.
In 1994, Spindel et al. proposed an implantable hearing device, in which a miniature permanent magnet is installed in an opening of the round window opposite the oval window (i.e., a connecting aperture) by the side of the external auditory ear canal maintained as it is, and an electronic coil for generating a magnetic field in response to a voice signal current is installed by the side of the mastoid, so as to transmit sound.
However, this scheme has drawbacks of poor practicability. Due to the relatively large distance between the electronic coil and the permanent magnet in the round window, a considerably large signal current is applied, thereby increasing the consumption of battery power.
In addition, US Patent Application Publication No. 2005/0020873 was proposed by Berrang et al. in 2005. As disclosed by this document, a vibrator is implanted in the bone above a vestibular organ adjacent to the inner ear, and includes a cylindrical housing and a multilayer piezoelectric structure, which has a diameter from 2 mm to 6 mm and is encased in the housing.
This scheme proposes another type of implantable hearing device, which designed based on a bone conduction mechanism. This type of implantable hearing device stimulates a region adjacent to the inner ear using a miniature vibrator in order to prevent a problem of attenuation by bone, which is caused by a preexisting bone conduction hearing device that stimulates the temporal bone using a bone conduction vibrator.
However, this scheme inevitably consumes a large amount of energy compared to the hearing device of Symphonix, Otologics, and St. Croix medical implantable middle hearing aid. Thus, it is not advantageous to apply this scheme to the middle ear implantable hearing device that has limited battery capacity.
In 1991, Engebreison et al. proposed a hearing device, which includes a totally-implantable middle ear body, an implantable microphone connected to the implantable middle ear body via an air tube instead of electric cords and a sound delivery coupler. With regard to the construction of this device, an implantable microphone is connected, through a tube, to a sound pickup coupler formed of a Titanium membrane, which is in contact with the malleus behind the ear drum. For sound output, a sound delivery coupler formed of a Titanium membrane is connected to the stapes through a tube, so as to transmit sound.
As peculiar characteristics of this device, the sound coupler is connected to the distal end of each tube so as to transmit sound pressure from the ear drum to the microphone inside the device body, and from the receiver inside the device body to the stapes. In fact, the application of the tube to transmit sound for higher sound transmission efficiency has been used in a process of inserting the tube into an earmold by connecting to the output of the receiver of a Behind The Ear (BTE) hearing device. Therefore, the application of the tube cannot be regarded as an inventive concept of this device.
When this device is realized as an actual middle ear implantable hearing device, there are drawbacks as follow.
Firstly, the structure of each sound coupler is too complicated, so that a miniature sound coupler cannot be easily fabricated or welded, thereby increasing manufacturing costs. This structure also requires very complicated and difficult processes, such as welding a small connector loop to the center of the membrane of a very thin Titanium film and hermetically sealing the entire structure.
Secondly, as another drawback of the hearing device of Engebreison et al., the ossicle of a patient should be cut such that an output from the receiver is not feed back to an adjacent sound coupler of the microphone.
Thirdly, the microphone and the microphone coupler should be prepared separately and the receiver and the receiver coupler should be set separately. Accordingly, the device is more bulky and complicated.
Fourthly, since the length of the sound tube connected to the receiver inside the body is increased to 5 cm or more, sound is generally attenuated, and especially, a high frequency range is significantly attenuated. Accordingly, this structure fails to provide clear sound to the user.
Fifthly, sound from the receiver is transmitted along the tube to vibrate the sound coupler membrane, which in turn vibrates the stapes. In view of the anatomical structure of the middle ear, the membrane of the sound coupler of the stapes cannot be sufficiently enlarged. Therefore, the sound coupler of the stapes does not greatly decrease high frequency transmission characteristics but greatly decreases low frequency transmission characteristics.
In 1993, Gilman was proposed another style of implantable hearing aid (U.S. Pat. No. 5,176,620) which using long liquid filled tube. In this method, a liquid filled tube is positioned between an orifice of cochlear and a subcutaneous receiver connected to amplifier. However, this method has same limitations as point out in the Engebreison et al.'s method, due to they use long liquid tube which severely reduces high frequency response. And Gilman conceptually proposed sound generator as a large size block in the drawing, but there were no explanation about the principle and mechanism of the receiver.
Recently, Vibrant MeDEL proposed another surgical scheme such as SoundBridge model, in which a floating mass vibrator by Ball et al. is wrapped in a fascia and a vibrator surface is brought into contact with the round window membrane. However, a relatively large region adjacent to the opening of the round window has to be cut in order to implant the vibrator having a diameter 2 mm in the cut region. Further, since the vibrator has to be fixed using only the fascia, it is difficult to guarantee that the vibrator stably remain in position for a long time after the operation.
The fixing process is insecure, and this scheme is rarely applicable to patients having severe hearing loss since the maximum driving force of the vibrator is dependent on the maximum driving force of the floating mass vibrator.
According to the paper by K. B. Huttenbrink of Germany, Otolaryngologic Clinic of North America, Vol. 34, No. 2, 2001, a piezoelectric vibrator having a housing is installed inside the squama tempolaris or the mastoid cavity of the skull, and a tube is connected from an output of the vibrator to the round window. This report proposed, for the first time, a method of using a balloon made of a thin film at the end of the tube when the tube is introduced into the round window.
Regarding that the vibrator is present inside the temporal bone rather than inside the tube, the general concept of this paper is similar to that proposed by Engebrison et al., in 1991, described as above. The tube is connected from the vibrator inside the temporal bone to the vibration-transmitting point along a long path. Therefore, a great amount of sound attenuation and distortion is followed, and the temporal bone has to be cut in when the operation is performed.
While this report proposed a concept that prepares a vessel having a thin film such as a balloon on the distal end of the tube and fills fluid into the vessel, details such as a shape, dimensions or material are not disclosed at all. The tube distal end of Huttenbrink disclosed by the document is configured as a balloon of a thin film, which exerts force in every direction and suspends from the preexisting tube. This configuration, however, may cause loss to the transmission of pressure.